An electronic control unit proposed in Japanese Patent Document No. JP-A-11-294252 includes an engine control microcomputer (engine MC) and a throttle control microcomputer (throttle MC). The engine MC contains a monitoring program for monitoring operation of the throttle MC. The monitoring program is independent from a base program that is programmed for various engine controls, and periodically executed by an interrupt.
The monitoring program compares a value calculated from data stored in the engine MC (e.g. fuel injection quantity) with a value calculated from data transmitted from the throttle MC. The values are calculated using the same operation. If the values match, the engine MC determines that the throttle MC operates normally.
Two watchdog timers are provided for the engine MC. One of the watchdog timers is used for monitoring the engine MC for proper execution of the base program. The other watchdog timer is used for monitoring the engine MC for proper execution of the monitoring program.
When the monitoring program is executed, it is determined whether subroutines contained in the monitoring program are executed in a predetermined sequence. If not, an output of watchdog pulses to the watchdog timer is stopped. In this case, the engine MC may not be able to monitor the throttle MC. Therefore, the watchdog timer for monitoring proper execution of the monitoring program stops power supply to a motor that drives a throttle valve.
In this device, the execution of the monitoring program is determined proper as long as the watchdog pulses are outputted to the watchdog timer. The engine MC cannot directly monitor the execution of the monitoring program, namely, the engine MC cannot directly monitor execution of required diagnosis. Thus, a fault cannot be detected if it occurs during the execution of the monitoring program under a condition that the watchdog pulses are outputted although the diagnosis is not performed.
An electronic control unit for controlling vehicular engine throttle valve performs various diagnoses, including verifications of parameter, data, and programs stored in a ROM. Therefore, monitoring executions of the diagnoses are very important for proper performance of the control.